During the period of this research in Costa Rica, 34 strains identified as belonging to the L. braziliensis complex have been isolated in cultures from humans. Antigens prepared from a human and an animal species of Leishmania (L. mexicans and L. hertigi) were used for immediate and delayed skin reactions in 26 persons with leishmaniasis, with very good correlation in 24. Of 33 persons with a positive culture, only 27 were positive by direct smears. Only 1 of 4 wild animals yielded a positive culture for a hemoflagellate but this was not identifid as Leishmania. Among 1712 sandflies captured, 23 species were identifed from the Pacific side and 11 from the Atlantic. The distribution of Lutzomyia olmeca, the vector of L. mexicana in other countries, has been completed in different areas of Costa Rica. L. longipalpis, the vector of kala-azar in other countries, was found for the first time in great numbers in a dry area on the Pacific side. Both Freund's complete adjuvant and a lysate of cultured parasites plus incomplete adjuvant arrested partially or totally the development of tumors (L. mexicana) but did not prevent a latent infection since viable parasites were found after several months in the apparently intact skin. The individual clinical pictures were not basically altered by simultaneous injection of L. mexicana and L. braziliensis or by the administration of BCG intracardiacally. L. mexicana maintained its virulence during numerous transfers in culture but L. braziliensis did not. Nude mice were quite resistant to L. braziliensis and their susceptibility to L. mexicana varied with the strain.